This invention relates to polymeric materials which stabilize particles including particles of colloidal size to prevent them from grouping together, i.e., agglomerating, when the particles are in suspension. In colloidal systems and especially liquid colloidal suspensions, the particles in suspension tend to group or stick together to form large groups of particles. This phenomenon is often referred to as agglomeration. The formation of large groups of particles destroys the substantially homogeneous distribution of the particles in suspension and essentially renders the suspension useless. This problem is particularly acute in suspensions which are used in light valves. In the operation of the light valve, a voltage is applied across the suspension. This voltage, because of the relative charges on or associated with the particles, can cause the particles to group together and form large groups of agglomerates. These masses prevent the proper functioning of the light valve and thereby destroy its utility.
There has been a need, therefore, to develop a material which would effectively act to prevent the suspended colloidal particles in a light valve from agglomerating.
Although the prior art is replete with patents pertaining to dispersants including polymeric materials for maintaining particles in suspension, these materials are either unsuitable for use in a light valve suspension or greatly inferior for such purpose to the polymers of the present invention. Descriptions of light valves that use a liquid suspension are given in U.S. Pat. No. 1,955,923 (Land), and in U.S. Pat. No. 3,708,219 (Forlini et al.). Basically they are devices which control the transmission of light.
In order to be suitable for use in a light valve suspension, a polymer should be soluble in the liquid suspending medium of the suspension. The polymer should furthermore be capable of associating with the surfaces of the suspended particles in order effectively to furnish steric protection from agglomeration particulary when particles are aligned under influence of an electric field, (a condition which drastically increases the tendency to agglomerate). Also the polymer should associate with the particles so that if the polymer is present when the particles are initially formed it will prevent the particles from growing too large and help to minimize formation of aggregates of particles during formation. The polymer should not attack the suspended particles so as to cause them to degrade and should not itself degrade at the temperature of use or at temperatures at which the suspension may be stored, lest its degradation products attack the suspended particles. Degradation causes a loss of the polymer's ability to impart steric protection and other benefits herein described. The polymer should preferably have a wide range of solubility so that, if desired, it can be dissolved in the polar liquids in which many of the particles used in light valves are initially formed, and also be soluble in relatively non-polar and low conductivity liquids used in operating light valves. The polymer should not coat the walls or the electrodes on the walls of the light valve, because a polymer that sticks to them creates a hazy appearance that destroys the clear view through the light valve and reduces the maximum light transmission or change of transmission attainable from the light valve. Further the polymer should improve the voltage characteristics of the suspension in that it should enable one to obtain a greater change in light transmission for a given voltage gradient applied across the suspension, than is possible if one employed nitrocellulose, the prior art polymer used in light valve suspensions by others. In this connection it is especially important and preferable to be able to do so at low frequencies e.g. 1000 Hertz or less, because electrical power usage is very much lower at low frequency than at higher activating frequencies.
Nitrocellulose, as previously mentioned, has been in use for a considerable length of time and does work to a certain extent in light valve suspensions. Although it does somewhat prevent agglomeration, nitrocellulose has the significant disadvantage of being highly subject to degradation at only moderately high temperatures. For example, nitrocellulose will degrade at or below 150.degree. F. At such temperatures nitrocellulose can break down and form nitrous and nitric acid and other degradation products and these can attack the particles in suspension. If the particles degrade when attacked by such acid or other degradation products the suspension will be destroyed. Nitrocellulose also has a further major disadvantage in that there are a limited number of suspending media in which it can be dissolved when it is used in light valves. These media are essentially limited to organic esters. For many functions esters are not the most desirable liquids in which to suspend the particles. Therefore, nitrocellulose has very serious chemical and physical drawbacks which are overcome by the polymeric materials of this invention. These polymers have much greater thermal stability than nitrocellulose and will not generally break down unless temperatures are reached that are far above the point at which nitrocellulose will form nitrous and nitric acid. In addition, they are soluble in many relatively nonconductive liquid suspending media in addition to esters.